1. Field of the Invention
This invention relates to a method for controlling or limiting the slip of a rail vehicle having at least two driving machines, each driving machine associated with a respective slip-limiting controller, wherein a nominal current value is varied as a function of the difference between a peripheral wheel speed derived from a driving axle of the respective machine and a presettable slip-limit value. The invention also relates to a slip-limiting control device for carrying out the method.
2. Description of the Prior Art
Prior art slip-limiting methods are described in the Swiss House Journal Brown Boveri Mitteilungen (1976), pages 160-167. In this journal, a speed controller for thyristor-controlled locomotives is specified in which the traction-motor current follows a nominal control value virtually without delay by means of phase-angle control. This nominal control value is supplied by a control loop including a Proportional-Integral-Derivative, or PID speed controller. The control loop is supplied at the input with a speed difference signal .DELTA.v= nominal speed value (entered at the driver's desk) - actual speed value (obtained from a trailing bogie) and a d.DELTA.v/dt signal formed by differentiation from .DELTA.v. In addition, in the control circuit a section is provided which slows down the build-up of the nominal current value to the extent that the change in acceleration of the train does not exceed a value of 0.6 m/s.sup.3 in order to avoid unpleasantly high accelerations for the passengers, to reduce load peaks in the supply system and to lower the overall energy consumption. The maximum current can be set by the vehicle driver at the driver's desk due to the fact that the nominal current value is limited at the speed controller.
In addition, an electronic anti-spin protection is provided which acts, via the current controller of the slipping bogie drive, on the traction-motor current when the slip between wheel and rail as a result of deteriorated conditions of adhesion exceeds 1.5% of the train speed. By slip, in this case the relative speed between wheel and rail is meant, that is to say the difference between the peripheral speed of the wheel and the train speed which would be accurately measureable, for example with a non-driven so-called running axle. In addition, a control loop exists which causes a rapid reduction in current when the acceleration of a wheelset exceeds 0.8 m/s.sup.2. This loop comes into effect when the conditions of adhesion suddenly deteriorate, for example when passing over a set of points.
However, modern traction vehicles are no longer equipped with a running axle. It has been considered adequate to base the determination of train speed on the axle having the smallest rotational speed or to determine it as an integral of the train acceleration and to calculate the train acceleration from tractionmotor current, train mass and the sum of the tractive resistances, with reference to which see also DE-OS No. 25 31 032 which corresponds to U.S. Pat. No. 4,065,975.
The frictional connection between the driving wheel and the rail changes within wide limits as a function of the coefficient of friction which, as a rule, is different for each axle, and thus the slip also changes. With increasing slip, the transferable traction force decreases after a maximum of adhesion has been exceeded and the driving wheels spin which causes increased wear to occur at the wheel and the rail and self-excited frictional vibrations between the traction motor and the wheelset and the two wheel discs on one axle. In order to transfer a maximum of traction force to the driving wheels, the slip in rail vehicles should be some km/h, that is to say all driven wheels should slip a little. On the other hand, the train speed cannot be determined as a function of the rotational speed of the axle if all driven wheels slip simultaneously.